Controlling Liquefaction of Natural Gas

ABSTRACT

A gas liquefaction process, especially for producing LNG, maintains product flow rate and temperature by controlling the refrigeration so that variation to reduce any difference between actual and required product temperatures is initiated before variation of the product flow rate to reduce any difference between actual and required flow rates.

BACKGROUND OF THE INVENTION

This invention relates to the field of control systems for production ofliquefied gas (LG), and more specifically, to a process and system whichcontrols LG production and LG temperature. It has particular but notexclusive application to liquefying natural gas (NG) to produceliquefied natural gas (LNG).

Systems for the liquefaction of natural gas (NG) by refrigeration inheat exchange means, especially using a multicomponent refrigerant, arein use throughout the world. Control of the LNG production process isimportant to operate a plant efficiently, especially when attempting tomeet demands for incremental production for downstream processing orwhen attempting to adjust to external process disturbances. Essentiallysimultaneous and independent control of both the LNG production flowrate and temperature is important for LNG plant operation. By fixing andmaintaining the LNG production rate, plant operators can adequately planand achieve desired production levels as required by the productshipping schedule. Maintaining the temperature of the LNG within aspecified range is important for downstream processing and theprevention of downstream equipment problems. Once regulatory control isachieved for the key variables, optimization strategies can be properlyimplemented. However, if regulatory control is not adequate, evenstandard day to day operation is adversely affected.

In typical NG liquefaction processes, natural gas is fed to the warm endof heat exchange means, having a liquefying section in which the naturalgas is liquefied and a subcooling section in which the liquefied naturalgas is subcooled, and the LNG outlet stream is withdrawn from the coldend of the heat exchange means. Some refrigeration duty in theliquefying section is provided by flashing a first refrigerant (“MRL”),provided by cooling in the heat exchange means the liquid portion of aphase separation of a multicomponent refrigerant (MR) and refrigerationduty in the subcooling section is provided by flashing a secondrefrigerant (“MRV”), provided by condensing in the heat exchange meansthe vapor portion of the MR phase separation. The remainder of therefrigeration duty in the liquefying section is provided by spent MRVfrom the liquefaction section. The refrigerants exiting the warm end ofthe heat exchanger means are combined, if not already mixed in theliquefaction section, compressed and precooled before return to the MRphase separation for recycle to the heat exchange means. A processhaving the aforementioned features is referred to herein as “a typicalNG liquefaction process”.

U.S. Pat. No. 5,791,160 (Mandler et at, corresponding to EP-A-0893665)describes a natural gas liquefaction control scheme where LNG productflow rate and temperature are simultaneously and independentlycontrolled by adjusting the amount of refrigeration. In the exemplifiedembodiments, the control variables (the ones having a set point that canbe changed by the operator) of a typical NG liquefaction process includeLNG product flow rate and temperature as well as the MRL/MRV ratio.Manipulated variables (the ones that are automatically controlled inresponse to operator setting of one or more of the control variables)include MR compressor speed and MR/LNG ratio. In this scheme the amountof refrigeration is adjusted after the actual LNG product flow rate hasbeen changed in response to a change in the LNG product flow rate setpoint.

U.S. Pat. No. 6,725,688 (Elion et al; corresponding to WO-A-01/81845)describes a modification of Mandler et a/with the object of maximizingpower utilization. LNG product temperature and MRL/MRV ratio areretained as controlled variables and the manipulated variable is LNG/MRLratio but LNG product flow rate cannot be independently set.

U.S. Patent Application Publication 2004/0255615 (Hupkes et al;corresponding to WO-A-2004/068049 & EP-A-1595101) describes the use ofan advanced process controller based on model-predictive control tocontrol a typical NG liquefaction process. The controller determinessimultaneous control actions for a set of manipulated variables in orderto optimize at least one of a set of parameters including the productionof liquefied product whilst controlling at least one of a set ofcontrolled variables. The set of manipulated variables includes MRL flowrate, MRV flow rate, MR composition, MR removal, MR compressor capacityand NG feed flow rate. The set of controlled variables includes thetemperature difference at the warm end of the main heat exchanger, anadjustable relating to the LNG temperature, the composition of therefrigerant entering the MR phase separator, the pressure in the shellof the main heat exchanger, and the pressure and liquid level in MRphase separator.

There is a need to develop a simple and robust control scheme thatallows control of LNG product temperature and flow rate withoutsubjecting the heat exchange means to thermal stresses and without theneed to manipulate the MR compressor and it is an object of the presentinvention to meet that need.

BRIEF SUMMARY OF THE INVENTION

A control system for typical NG liquefaction processes has been devisedin which the thermal stress on the heat exchange means is limited andthe need to manipulate the MR compressor can be avoided by controllingthe refrigeration so that variation to reduce any difference betweenactual and required LNG temperature is initiated before variation of theLNG product flow rate to reduce any difference between actual andrequired LNG flow rate. Accordingly, refrigeration leads LG production.The invention has particular, but not exclusive, application to atypical NG liquefaction process in which the controlled variables areLNG temperature, LNG flow rate and either heat exchanger warm endtemperature difference (“WETD”) or heat exchanger mid-point temperature(“MPT”) and the manipulated variables are MRL and MRV flow rates.However, the invention is not restricted to the control of NGliquefaction processes but is more generally applicable to gasliquefaction, e.g. of hydrocarbon mixtures.

In one of its broadest aspects, the invention provides a method ofmaintaining at an adjustable predetermined flow rate value and at anadjustable predetermined temperature value the liquefied gas (“LG”)outlet stream of a gas liquefaction in which a gas feed is liquefied byrefrigeration in heat exchange means, comprising the steps of:

setting the predetermined flow rate value for the LG outlet stream andcomparing said value with the actual LG flow rate;

setting the predetermined temperature value for the LG outlet stream andcomparing said LNG temperature value with the actual LG temperature;

varying the refrigeration provided by said heat exchange means inresponse to said LG flow rate and LG temperature comparisons to reduceany differences, characterized in that the refrigeration is varied toreduce any LG temperature difference before variation of the LG flowrate to reduce any LG flow rate difference. Thus, this aspect allows theLG flow rate and temperature to be independently set and refrigerationto be correspondingly adjusted to meet the set requirements with limitedthermal stress on the heat exchange means. However, the control systemconcept of the invention is applicable to LG liquefaction processes inwhich the LG flow rate and temperature requirements are constant butfrom time to time some variation is required to the actual values inorder to compensate for a change in other parameters, such as NG feedtemperature and composition, MR composition, ambient air temperature,cooling water temperature, atmospheric pressure etc., that has causedthe actual value to deviate from the required value.

The invention also provides a control system for maintaining at anadjustable predetermined flow rate value and at an adjustablepredetermined temperature value the liquefied gas (“LG”) outlet streamof a gas liquefaction in which a gas feed is liquefied by refrigerationin heat exchange means, comprising:

means for setting the predetermined flow rate value for the LG outletstream and comparing said value with the actual LG flow rate;

means for varying the actual LG product flow rate;

means for setting the predetermined temperature value for the LG outletstream and comparing said LNG temperature value with the actual LGtemperature; and

means for varying the refrigeration provided by said heat exchange meansin response to said LG flow rate and LG temperature comparisons toreduce any differences,

characterized in that means for varying the actual LG product flow rateis not adjusted until the refrigeration has been adjusted to reduce anyLG temperature difference.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic flow diagram of a mixed refrigerant LNG plantprocess of a first exemplary embodiment of the present invention.

FIG. 2 is a schematic flow diagram of a mixed refrigerant LNG plantprocess of a second exemplary embodiment of the present invention.

FIG. 3 is a schematic flow diagram of a mixed refrigerant LNG plantprocess of a third exemplary embodiment of the present invention.

FIG. 4 is a schematic flow diagram of a mixed refrigerant LNG plantprocess of a fourth exemplary embodiment of the present invention.

FIG. 5 is a schematic flow diagram of a mixed refrigerant LNG plantprocess of a fifth exemplary embodiment of the present invention.

FIG. 6 is a schematic flow diagram of a modification of the mixedrefrigerant LNG plant process of FIG. 3.

FIG. 7 is a schematic flow diagram of a comparative mixed refrigerantLNG plant process.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the control of liquefaction of gas,especially natural gas, in a manner that maintains the LG product at arequired flow rate and temperature with limited thermal stress on theheat exchange means even when the LG flow rate and/or temperaturerequirements have been changed. The invention resides in the manner inwhich refrigeration is changed by manipulated variables.

In one broad aspect, the invention provides a method of maintaining atan adjustable predetermined flow rate value and at an adjustablepredetermined temperature value the liquefied gas (“LG”) outlet streamof a gas liquefaction in which a gas feed is liquefied by refrigerationin heat exchange means, comprising the steps of:

setting the predetermined flow rate value for the LG outlet stream andcomparing said value with the actual LG flow rate;

setting the predetermined temperature value for the LG outlet stream andcomparing said LG temperature value with the actual LG temperature; and

varying the refrigeration provided by said heat exchange means inresponse to said LG flow rate and LG temperature comparisons to reduceany differences,

characterized in that the refrigeration is varied to reduce any LGtemperature difference before variation of the LG flow rate to reduceany LG flow rate difference.

In a corresponding apparatus aspect, the invention also provides acontrol system for maintaining at an adjustable predetermined flow ratevalue and at an adjustable predetermined temperature value the liquefiedgas (“LG”) outlet stream of a gas liquefaction in which a gas feed isliquefied by refrigeration in heat exchange means, comprising:

means for setting the predetermined flow rate value for the LG outletstream and comparing said value with the actual LG flow rate;

means for varying the actual LG product flow rate;

means for setting the predetermined temperature value for the LG outletstream and comparing said LNG temperature value with the actual LGtemperature; and

means for varying the refrigeration provided by said heat exchange meansin response to said LG flow rate and LG temperature comparisons toreduce any differences,

characterized in that the means for varying the actual LG product flowrate is not adjusted until the refrigeration has been adjusted to reduceany LG temperature difference.

In another broad aspect, the invention also provides a method ofmaintaining at a predetermined flow rate value and at a predeterminedtemperature value the liquefied gas (“LG”) outlet stream of a gasliquefaction in which a gas feed is liquefied by refrigeration in heatexchange means, comprising the steps of:

comparing said predetermined LG flow rate value with the actual LG flowrate;

comparing said predetermined LG temperature value with the actual LGtemperature; and

varying the refrigeration provided by said heat exchange means inresponse to said LG flow rate and LG temperature comparisons to reduceany differences, characterized in that the refrigeration is varied toreduce any LG temperature difference before variation of the LNG flowrate to reduce any LG flow rate difference.

In a corresponding apparatus aspect, the invention also provides acontrol system for maintaining at a predetermined flow rate value and ata predetermined temperature value the liquefied gas (“LG”) outlet streamof a gas liquefaction in which a gas feed is liquefied by refrigerationin heat exchange means, comprising:

means for comparing said predetermined LG flow rate value with theactual LG flow rate;

means for comparing said predetermined LG temperature value with theactual LG temperature; and

means for varying the actual LG product flow rate;

means for varying the refrigeration provided by said heat exchange meansin response to said LG flow rate and LG temperature comparisons toreduce any differences,

characterized in that the means for varying the LNG flow rate is notadjusted until the refrigeration has been varied to reduce any LGtemperature difference.

The invention has particular application to typical NG liquefactionprocesses and in a preferred embodiment provides a method of maintainingat an adjustable predetermined flow rate value and at an adjustablepredetermined temperature value the liquefied natural gas (“LNG”) outletstream of a natural gas liquefaction using heat exchange means, having awarm end to which the natural gas is fed, a liquefying section in whichthe natural gas is liquefied, a subcooling section in which theliquefied natural gas is subcooled and a cold end from which said LNGoutlet stream is withdrawn, in which refrigeration duty is provided inthe liquefying section by a first refrigerant (“MRL”) cooled in saidheat exchange means and supplied for refrigeration duty at an MRL flowrate and in the subcooling section by a second refrigerant (“MRV”)cooled in said heat exchange means and supplied for refrigeration dutyat an MRV flow rate, comprising the steps of:

setting the predetermined flow rate value for the LNG outlet stream andcomparing said value with the actual LNG flow rate;

setting the predetermined temperature value for the LNG outlet streamand comparing said LNG temperature value with the actual LNGtemperature;

setting a predetermined value of (i) the temperature difference betweenspent refrigerant leaving the warm end of the heat exchange means and astream entering said warm end selected from MRL, MRV and the natural gasfeed (“warm end temperature difference value”) or (ii) the temperatureof a stream at a location between the liquefying and subcooling sectionsof the heat exchanger means (“mid-point temperature”) and comparing samewith the actual warm end temperature difference or actual mid-pointtemperature respectively;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates, one of the MRL and MRV flow rates;

varying the other of the MRV and MRL flow rates to maintain an MRL/MRVratio, which ratio is determined by one of (a) the difference betweenthe actual and predetermined LNG temperatures and (b) the differencebetween the actual and predetermined warm end temperature differences ormid-point temperatures; and

varying, by an amount corresponding to the other of (b) the differencebetween the actual and predetermined warm end temperature differences ormid-point temperatures and (a) the difference between the actual andpredetermined LNG temperatures, the actual LNG flow rate.

In a corresponding apparatus aspect, the invention provides a controlsystem for maintaining at an adjustable predetermined flow rate valueand at an adjustable predetermined temperature value the liquefiednatural gas (“LNG”) outlet stream of a natural gas liquefaction usingheat exchange means, having a warm end to which the natural gas is fed,a liquefying section in which the natural gas is liquefied, a subcoolingsection in which the liquefied natural gas is subcooled and a cold endfrom which said LNG outlet stream is withdrawn, in which refrigerationduty is provided in the liquefying section by a first refrigerant(“MRL”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRL flow rate and in the subcooling section bya second refrigerant (“MRV”) cooled in said heat exchange means andsupplied for refrigeration duty at an MRV flow rate, comprising:

means for setting the predetermined flow rate value for the LNG outletstream and comparing said value with the actual LNG flow rate;

means for setting the predetermined temperature value for the LNG outletstream and comparing said LNG temperature value with the actual LNGtemperature;

means for setting a predetermined value of (i) the temperaturedifference between spent refrigerant leaving the warm end of the heatexchange means and a stream entering said warm end selected from MRL,MRV and the natural gas feed (“warm end temperature difference value”)or (ii) the temperature of a stream at a location between the liquefyingand subcooling sections of the heat exchanger means (“mid-pointtemperature”) and comparing same with the actual warm end temperaturedifference or actual mid-point temperature respectively;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates, one of the MRL and MRV flowrates;

means for varying the other of the MRV and MRL flow rates to maintain anMRL/MRV ratio, which ratio is determined by one of (a) the differencebetween the actual and predetermined LNG temperatures and (b) thedifference between the actual and predetermined warm end temperaturedifferences or mid-point temperatures; and

means for varying, by an amount corresponding to the other of (b) thedifference between the actual and predetermined warm end temperaturedifferences or mid-point temperatures and (a) the difference between theactual and predetermined LNG temperatures, the actual LNG flow rate.

Another preferred embodiment of the invention provides a method ofmaintaining at a predetermined flow rate value and at a predeterminedtemperature value the liquefied natural gas (“LNG”) outlet stream of anatural gas liquefaction using heat exchange means, having a warm end towhich the natural gas is fed, a liquefying section in which the naturalgas is liquefied, a subcooling section in which the liquefied naturalgas is subcooled and a cold end from which said LNG outlet stream iswithdrawn, in which refrigeration duty is provided in the liquefyingsection by a first refrigerant (“MRL”) cooled in said heat exchangemeans and supplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising the steps of:

comparing said predetermined LNG flow rate value with the actual LNGflow rate;

comparing said predetermined LNG temperature value with the actual LNGtemperature;

comparing a predetermined value of (i) the temperature differencebetween spent refrigerant leaving the warm end of the heat exchangemeans and a stream entering said warm end selected from MRL, MRV and thenatural gas feed (“warm end temperature difference value”) or (ii) thetemperature of a stream at a location between the liquefying andsubcooling sections of the heat exchanger means (“mid-pointtemperature”) with the actual warm end temperature difference or actualmid-point temperature respectively;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates, one of the MRL and MRV flow rates;

varying the other of the MRV and MRL flow rates to maintain an MRL/MRVratio, which ratio is determined by one of (a) the difference betweenthe actual and predetermined LNG temperatures and (b) the differencebetween the actual and predetermined warm end temperature differences ormid-point temperatures; and

varying, by an amount corresponding to the other of (b) the differencebetween the actual and predetermined warm end temperature differences ormid-point temperatures and (a) the difference between the actual andpredetermined LNG temperatures, the actual LNG flow rate.

In a corresponding apparatus embodiment, the invention provides acontrol system for maintaining at a predetermined flow rate value and ata predetermined temperature value the liquefied natural gas (“LNG”)outlet stream of a natural gas liquefaction using heat exchange means,having a warm end to which the natural gas is fed, a liquefying sectionin which the natural gas is liquefied, a subcooling section in which theliquefied natural gas is subcooled and a cold end from which said LNGoutlet stream is withdrawn, in which refrigeration duty is provided inthe liquefying section by a first refrigerant (“MRL”) cooled in saidheat exchange means and supplied for refrigeration duty at an MRL flowrate and in the subcooling section by a second refrigerant (“MRV”)cooled in said heat exchange means and supplied for refrigeration dutyat an MRV flow rate, comprising:

means for comparing said predetermined LNG flow rate value with theactual LNG flow rate;

means for comparing said predetermined LNG temperature value with theactual LNG temperature;

means for comparing a predetermined value of (i) the temperaturedifference between spent refrigerant leaving the warm end of the heatexchange means and a stream entering said warm end selected from MRL,MRV and the natural gas feed (“warm end temperature difference value”)or (ii) the temperature of a stream at a location between the liquefyingand subcooling sections of the heat exchanger means (“mid-pointtemperature”) with the actual warm end temperature difference or actualmid-point temperature respectively;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates, one of the MRL and MRV flowrates;

means for varying the other of the MRV and MRL flow rates to maintain anMRL/MRV ratio, which ratio is determined by one of (a) the differencebetween the actual and predetermined LNG temperatures and (b) thedifference between the actual and predetermined warm end temperaturedifferences or mid-point temperatures; and

means for varying, by an amount corresponding to the other of (b) thedifference between the actual and predetermined warm end temperaturedifferences or mid-point temperatures and (a) the difference between theactual and predetermined LNG temperatures, the actual LNG flow rate.

In accordance with an embodiment illustrated in FIG. 1, the warm endtemperature difference value is predetermined; the MRL flow rate isadjusted in response to the difference between actual and predeterminedLNG product flow rates and hence the LNG/MRL ratio changed; the requiredMRL/MRV ratio is adjusted in response to the difference between actualand predetermined warm end temperature difference value and the MRV flowrate adjusted to achieve that ratio; and the actual flow rate isadjusted in response to the difference between actual and predeterminedLNG product temperatures.

Thus, in accordance with the embodiment illustrated in FIG. 1, theinvention provides a method of maintaining at an adjustablepredetermined flow rate value and at an adjustable predeterminedtemperature value the liquefied natural gas (“LNG”) outlet stream of anatural gas liquefaction using heat exchange means, having a warm end towhich the natural gas is fed, a liquefying section in which the naturalgas is liquefied, a subcooling section in which the liquefied naturalgas is subcooled and a cold end from which said LNG outlet stream iswithdrawn, in which refrigeration duty is provided in the liquefyingsection by a first refrigerant (“MRL”) cooled in said heat exchangemeans and supplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising the steps of:

setting the predetermined flow rate value for the LNG outlet stream andcomparing said value with the actual LNG flow rate;

setting the predetermined temperature value for the LNG outlet streamand comparing said LNG temperature value with the actual LNGtemperature;

setting a predetermined value of the temperature difference betweenspent refrigerant leaving the warm end of the heat exchange means and astream entering said warm end selected from MRL, MRV and the natural gasfeed (“warm end temperature difference value”) and comparing same withthe actual warm end temperature difference;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates, the MRL flow rate;

varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio isdetermined by the difference between the actual and predetermined warmend temperature differences; and

varying, by an amount corresponding to the difference between the actualand predetermined LNG temperatures, the actual LNG flow rate.

In a corresponding apparatus embodiment, the invention provides acontrol system for maintaining at an adjustable predetermined flow ratevalue and at an adjustable predetermined temperature value the liquefiednatural gas (“LNG”) outlet stream of a natural gas liquefaction usingheat exchange means, having a warm end to which the natural gas is fed,a liquefying section in which the natural gas is liquefied, a subcoolingsection in which the liquefied natural gas is subcooled and a cold endfrom which said LNG outlet stream is withdrawn, in which refrigerationduty is provided in the liquefying section by a first refrigerant(“MRL”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRL flow rate and in the subcooling section bya second refrigerant (“MRV”) cooled in said heat exchange means andsupplied for refrigeration duty at an MRV flow rate, comprising:

means for setting the predetermined flow rate value for the LNG outletstream and comparing said value with the actual LNG flow rate;

means for setting the predetermined temperature value for the LNG outletstream and comparing said LNG temperature value with the actual LNGtemperature;

means for setting a predetermined value of the temperature differencebetween spent refrigerant leaving the warm end of the heat exchangemeans and a stream entering said warm end selected from MRL, MRV and thenatural gas feed (“warm end temperature difference value”) and comparingsame with the actual warm end temperature difference;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates, the MRL flow rate;

means for varying the MRV flow rate to maintain an MRL/MRV ratio, whichratio is determined by the difference between the actual andpredetermined warm end temperature differences; and

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG temperatures, the actual LNG flow rate.

Also in accordance with an embodiment illustrated in FIG. 1, theinvention provides a method of maintaining at a predetermined flow ratevalue and at a predetermined temperature value the liquefied natural gas(“LNG”) outlet stream of a natural gas liquefaction using heat exchangemeans, having a warm end to which the natural gas is fed, a liquefyingsection in which the natural gas is liquefied, a subcooling section inwhich the liquefied natural gas is subcooled and a cold end from whichsaid LNG outlet stream is withdrawn, in which refrigeration duty isprovided in the liquefying section by a first refrigerant (“MRL”) cooledin said heat exchange means and supplied for refrigeration duty at anMRL flow rate and in the subcooling section by a second refrigerant(“MRV”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRV flow rate, comprising the steps of:

comparing said predetermined LNG flow rate value with the actual LNGflow rate;

comparing said predetermined LNG temperature value with the actual LNGtemperature;

comparing a predetermined value of the temperature difference betweenspent refrigerant leaving the warm end of the heat exchange means and astream entering said warm end selected from MRL, MRV and the natural gasfeed (“warm end temperature difference value”) with the actual warm endtemperature difference;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates, the MRL flow rate;

varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio isdetermined by the difference between the actual and predetermined warmend temperature differences; and

varying, by an amount corresponding to the difference between the actualand predetermined LNG temperatures, the actual LNG flow rate.

In a corresponding apparatus embodiment, the invention provides acontrol system for maintaining at a predetermined flow rate value and ata predetermined temperature value the liquefied natural gas (“LNG”)outlet stream of a natural gas liquefaction using heat exchange means,having a warm end to which the natural gas is fed, a liquefying sectionin which the natural gas is liquefied, a subcooling section in which theliquefied natural gas is subcooled and a cold end from which said LNGoutlet stream is withdrawn, in which refrigeration duty is provided inthe liquefying section by a first refrigerant (“MRL”) cooled in saidheat exchange means and supplied for refrigeration duty at an MRL flowrate and in the subcooling section by a second refrigerant (“MRV”)cooled in said heat exchange means and supplied for refrigeration dutyat an MRV flow rate, comprising:

means for comparing said predetermined LNG flow rate value with theactual LNG flow rate;

means for comparing said predetermined LNG temperature value with theactual LNG temperature;

means for comparing a predetermined value of the temperature differencebetween spent refrigerant leaving the warm end of the heat exchangemeans and a stream entering said warm end selected from MRL, MRV and thenatural gas feed (“warm end temperature difference value”) with theactual warm end temperature difference;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates, the MRL flow rate;

means for varying the MRV flow rate to maintain an MRL/MRV ratio, whichratio is determined by the difference between the actual andpredetermined warm end temperature differences; and

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG temperatures, the actual LNG flow rate.

In accordance with an embodiment illustrated in FIG. 2, the warm endtemperature difference value is predetermined; the MRL flow rate isadjusted in response to the difference between actual and predeterminedLNG product flow rates and hence the LNG/MRL ratio changed; the requiredMRL/MRV ratio is adjusted in response to the difference between actualand predetermined LNG product temperatures and the MRV flow rateadjusted to achieve that ratio; and the actual flow rate is adjusted inresponse to the difference between actual and predetermined warm endtemperature difference values.

Thus, in accordance with an embodiment illustrated in FIG. 2, theinvention provides a method of maintaining at an adjustablepredetermined flow rate value and at an adjustable predeterminedtemperature value the liquefied natural gas (“LNG”) outlet stream of anatural gas liquefaction using heat exchange means, having a warm end towhich the natural gas is fed, a liquefying section in which the naturalgas is liquefied, a subcooling section in which the liquefied naturalgas is subcooled and a cold end from which said LNG outlet stream iswithdrawn, in which refrigeration duty is provided in the liquefyingsection by a first refrigerant (“MRL”) cooled in said heat exchangemeans and supplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising the steps of:

setting the predetermined flow rate value for the LNG outlet stream andcomparing said value with the actual LNG flow rate;

setting the predetermined temperature value for the LNG outlet streamand comparing said LNG temperature value with the actual LNGtemperature;

setting a predetermined value of the temperature difference betweenspent refrigerant leaving the warm end of the heat exchange means and astream entering said warm end selected from MRL, MRV and the natural gasfeed (“warm end temperature difference value”) and comparing same withthe actual warm end temperature difference;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates, the MRL flow rate;

varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio isdetermined by the difference between the actual and predetermined LNGtemperatures; and

varying, by an amount corresponding to the difference between the actualand predetermined warm end temperature differences, the actual LNG flowrate.

In a corresponding apparatus embodiment, the invention provides acontrol system for maintaining at an adjustable predetermined flow ratevalue and at an adjustable predetermined temperature value the liquefiednatural gas (“LNG”) outlet stream of a natural gas liquefaction usingheat exchange means, having a warm end to which the natural gas is fed,a liquefying section in which the natural gas is liquefied, a subcoolingsection in which the liquefied natural gas is subcooled and a cold endfrom which said LNG outlet stream is withdrawn, in which refrigerationduty is provided in the liquefying section by a first refrigerant(“MRL”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRL flow rate and in the subcooling section bya second refrigerant (“MRV”) cooled in said heat exchange means andsupplied for refrigeration duty at an MRV flow rate, comprising:

means for setting the predetermined flow rate value for the LNG outletstream and comparing said value with the actual LNG flow rate;

means for setting the predetermined temperature value for the LNG outletstream and comparing said LNG temperature value with the actual LNGtemperature;

means for setting a predetermined value of the temperature differencebetween spent refrigerant leaving the warm end of the heat exchangemeans and a stream entering said warm end selected from MRL, MRV and thenatural gas feed (“warm end temperature difference value”) and comparingsame with the actual warm end temperature difference;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates, the MRL flow rate;

means for varying the MRV flow rate to maintain an MRL/MRV ratio, whichratio is determined by the difference between the actual andpredetermined LNG temperatures; and

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined warm end temperature differences, theactual LNG flow rate.

Also in accordance with an embodiment illustrated in FIG. 2, theinvention provides a method of maintaining at a predetermined flow ratevalue and at a predetermined temperature value the liquefied natural gas(“LNG”) outlet stream of a natural gas liquefaction using heat exchangemeans, having a warm end to which the natural gas is fed, a liquefyingsection in which the natural gas is liquefied, a subcooling section inwhich the liquefied natural gas is subcooled and a cold end from whichsaid LNG outlet stream is withdrawn, in which refrigeration duty isprovided in the liquefying section by a first refrigerant (“MRL”) cooledin said heat exchange means and supplied for refrigeration duty at anMRL flow rate and in the subcooling section by a second refrigerant(“MRV”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRV flow rate, comprising the steps of:

comparing said predetermined LNG flow rate value with the actual LNGflow rate;

comparing said predetermined LNG temperature value with the actual LNGtemperature;

comparing a predetermined value of the temperature difference betweenspent refrigerant leaving the warm end of the heat exchange means and astream entering said warm end selected from MRL, MRV and the natural gasfeed (“warm end temperature difference value”) with the actual warm endtemperature difference;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates, the MRL flow rate;

varying the MRV flow rate to maintain an MRL/MRV ratio, which ratiodetermined by the difference between the actual and predetermined LNGtemperatures; and

varying, by an amount corresponding to the difference between the actualand predetermined warm end temperature differences, the actual LNG flowrate.

In accordance with a corresponding apparatus embodiment, the inventionprovides a control system for maintaining at a predetermined flow ratevalue and at a predetermined temperature value the liquefied natural gas(“LNG”) outlet stream of a natural gas liquefaction using heat exchangemeans, having a warm end to which the natural gas is fed, a liquefyingsection in which the natural gas is liquefied, a subcooling section inwhich the liquefied natural gas is subcooled and a cold end from whichsaid LNG outlet stream is withdrawn, in which refrigeration duty isprovided in the liquefying section by a first refrigerant (“MRL”) cooledin said heat exchange means and supplied for refrigeration duty at anMRL flow rate and in the subcooling section by a second refrigerant(“MRV”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRV flow rate, comprising:

means for comparing said predetermined LNG flow rate value with theactual LNG flow rate;

means for comparing said predetermined LNG temperature value with theactual LNG temperature;

means for comparing a predetermined value of the temperature differencebetween spent refrigerant leaving the warm end of the heat exchangemeans and a stream entering said warm end selected from MRL, MRV and thenatural gas feed (“warm end temperature difference value”) with theactual warm end temperature difference;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates, the MRL flow rate;

means for varying the MRV flow rate to maintain an MRL/MRV ratio, whichratio determined by the difference between the actual and predeterminedLNG temperatures; and

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined warm end temperature differences, theactual LNG flow rate.

In accordance with an embodiment illustrated in FIG. 3, the warm endtemperature difference value is predetermined; the MRL flow rate isadjusted in response to the difference between actual and predeterminedLNG product flow rates and hence the LNG/MRL ratio changed; the requiredMRL/MRV ratio is adjusted in response to the difference between actualand predetermined LNG product temperatures and the MRV flow rateadjusted to achieve that ratio; and the actual flow rate is adjusted inresponse to both the difference between actual and predetermined warmend temperature difference values and the actual MRL flow rate.

Thus, in accordance with an embodiment illustrated in FIG. 3, theinvention provides a method of maintaining at an adjustablepredetermined flow rate value and at an adjustable predeterminedtemperature value the liquefied natural gas (“LNG”) outlet stream of anatural gas liquefaction using heat exchange means, having a warm end towhich the natural gas is fed, a liquefying section in which the naturalgas is liquefied, a subcooling section in which the liquefied naturalgas is subcooled and a cold end from which said LNG outlet stream iswithdrawn, in which refrigeration duty is provided in the liquefyingsection by a first refrigerant (“MRL”) cooled in said heat exchangemeans and supplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising the steps of:

setting the predetermined flow rate value for the LNG outlet stream andcomparing said value with the actual LNG flow rate;

setting the predetermined temperature value for the LNG outlet streamand comparing said LNG temperature value with the actual LNGtemperature;

setting a predetermined value of the temperature difference betweenspent refrigerant leaving the warm end of the heat exchange means and astream entering said warm end selected from MRL, MRV and the natural gasfeed (“warm end temperature difference value”) and comparing same withthe actual warm end temperature difference;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates, the MRL flow rate;

varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio isdetermined by the difference between the actual and predetermined LNGtemperatures; and

varying, by an amount corresponding to the difference between the actualand predetermined warm end temperature differences multiplied by a valuedependent on the actual MRL flow rate, the actual LNG flow rate.

In accordance with a corresponding apparatus embodiment, the inventionprovides a control system for maintaining at an adjustable predeterminedflow rate value and at an adjustable predetermined temperature value theliquefied natural gas (“LNG”) outlet stream of a natural gasliquefaction using heat exchange means, having a warm end to which thenatural gas is fed, a liquefying section in which the natural gas isliquefied, a subcooling section in which the liquefied natural gas issubcooled and a cold end from which said LNG outlet stream is withdrawn,in which refrigeration duty is provided in the liquefying section by afirst refrigerant (“MRL”) cooled in said heat exchange means andsupplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising:

means for setting the predetermined flow rate value for the LNG outletstream and comparing said value with the actual LNG flow rate;

means for setting the predetermined temperature value for the LNG outletstream and comparing said LNG temperature value with the actual LNGtemperature;

means for setting a predetermined value of the temperature differencebetween spent refrigerant leaving the warm end of the heat exchangemeans and a stream entering said warm end selected from MRL, MRV and thenatural gas feed (“warm end temperature difference value”) and comparingsame with the actual warm end temperature difference;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates, the MRL flow rate;

means for varying the MRV flow rate to maintain an MRL/MRV ratio, whichratio is determined by the difference between the actual andpredetermined LNG temperatures; and

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined warm end temperature differences multipliedby a value dependent on the actual MRL flow rate, the actual LNG flowrate.

Also in accordance with an embodiment illustrated in FIG. 3, theinvention provides a method of maintaining at a predetermined flow ratevalue and at a predetermined temperature value the liquefied natural gas(“LNG”) outlet stream of a natural gas liquefaction using heat exchangemeans, having a warm end to which the natural gas is fed, a liquefyingsection in which the natural gas is liquefied, a subcooling section inwhich the liquefied natural gas is subcooled and a cold end from whichsaid LNG outlet stream is withdrawn, in which refrigeration duty isprovided in the liquefying section by a first refrigerant (“MRL”) cooledin said heat exchange means and supplied for refrigeration duty at anMRL flow rate and in the subcooling section by a second refrigerant(“MRV”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRV flow rate, comprising the steps of:

comparing said predetermined LNG flow rate value with the actual LNGflow rate;

comparing said predetermined LNG temperature value with the actual LNGtemperature;

comparing a predetermined value of the temperature difference betweenspent refrigerant leaving the warm end of the heat exchange means and astream entering said warm end selected from MRL, MRV and the natural gasfeed (“warm end temperature difference value”) with the actual warm endtemperature difference;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates, the MRL flow rate;

varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio isdetermined by the difference between the actual and predetermined LNGtemperatures; and

varying, by an amount corresponding to the difference between the actualand predetermined warm end temperature differences multiplied by a valuedependent on the actual MRL flow rate, the actual LNG flow rate.

In accordance with a corresponding apparatus embodiment, the inventionprovides a control system for maintaining at a predetermined flow ratevalue and at a predetermined temperature value the liquefied natural gas(“LNG”) outlet stream of a natural gas liquefaction using heat exchangemeans, having a warm end to which the natural gas is fed, a liquefyingsection in which the natural gas is liquefied, a subcooling section inwhich the liquefied natural gas is subcooled and a cold end from whichsaid LNG outlet stream is withdrawn, in which refrigeration duty isprovided in the liquefying section by a first refrigerant (“MRL”) cooledin said heat exchange means and supplied for refrigeration duty at anMRL flow rate and in the subcooling section by a second refrigerant(“MRV”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRV flow rate, comprising:

means for comparing said predetermined LNG flow rate value with theactual LNG flow rate;

means for comparing said predetermined LNG temperature value with theactual LNG temperature;

means for comparing a predetermined value of the temperature differencebetween spent refrigerant leaving the warm end of the heat exchangemeans and a stream entering said warm end selected from MRL, MRV and thenatural gas feed (“warm end temperature difference value”) with theactual warm end temperature difference;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates, the MRL flow rate;

means for varying the MRV flow rate to maintain an MRL/MRV ratio, whichratio is determined by the difference between the actual andpredetermined LNG temperatures; and

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined warm end temperature differences multipliedby a value dependent on the actual MRL flow rate, the actual LNG flowrate.

In accordance with an embodiment illustrated in FIG. 4, the mid-pointtemperature difference value is predetermined; the MRL flow rate isadjusted in response to the difference between actual and predeterminedLNG product flow rates and hence the LNG/MRL ratio changed; the requiredMRL/MRV ratio is adjusted in response to the difference between actualand predetermined mid-point temperatures and the MRV flow rate adjustedto achieve that ratio; and the actual flow rate is adjusted in responseto the difference between actual and predetermined LNG producttemperatures. Preferably, the warm end temperature difference ispredetermined and the difference between actual and predetermined warmend temperature differences used as an override control of the MRL flowrate when said difference exceeds a predetermined value.

Thus, in accordance with an embodiment illustrated in FIG. 4, theinvention provides a method of maintaining at an adjustablepredetermined flow rate value and at an adjustable predeterminedtemperature value the liquefied natural gas (“LNG”) outlet stream of anatural gas liquefaction using heat exchange means, having a warm end towhich the natural gas is fed, a liquefying section in which the naturalgas is liquefied, a subcooling section in which the liquefied naturalgas is subcooled and a cold end from which said LNG outlet stream iswithdrawn, in which refrigeration duty is provided in the liquefyingsection by a first refrigerant (“MRL”) cooled in said heat exchangemeans and supplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising the steps of:

setting the predetermined flow rate value for the LNG outlet stream andcomparing said value with the actual LNG flow rate;

setting the predetermined temperature value for the LNG outlet streamand comparing said LNG temperature value with the actual LNGtemperature;

setting a predetermined value of the temperature difference betweenspent refrigerant leaving the warm end of the heat exchange means and astream entering said warm end selected from MRL, MRV and the natural gasfeed (“warm end temperature difference value”) and comparing same withthe actual warm end temperature difference;

setting a predetermined value of the temperature of a stream at alocation between the liquefying and subcooling sections of the heatexchanger means (“mid-point temperature”) and comparing same with theactual mid-point temperature;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates and also, when the difference betweenactual and predetermined warm end temperature differences exceeds athreshold value, to said difference between warm end temperaturedifferences, MRL flow rate;

varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio isdetermined by the difference between the actual and predeterminedmid-point temperatures; and

varying, by an amount corresponding to the difference between thedifference between the actual and predetermined LNG temperatures, theactual LNG flow rate.

In accordance with a corresponding apparatus embodiment, the inventionprovides a control system for maintaining at an adjustable predeterminedflow rate value and at an adjustable predetermined temperature value theliquefied natural gas (“LNG”) outlet stream of a natural gasliquefaction using heat exchange means, having a warm end to which thenatural gas is fed, a liquefying section in which the natural gas isliquefied, a subcooling section in which the liquefied natural gas issubcooled and a cold end from which said LNG outlet stream is withdrawn,in which refrigeration duty is provided in the liquefying section by afirst refrigerant (“MRL”) cooled in said heat exchange means andsupplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising:

means for setting the predetermined flow rate value for the LNG outletstream and comparing said value with the actual LNG flow rate;

means for setting the predetermined temperature value for the LNG outletstream and comparing said LNG temperature value with the actual LNGtemperature;

means for setting a predetermined value of the temperature differencebetween spent refrigerant leaving the warm end of the heat exchangemeans and a stream entering said warm end selected from MRL, MRV and thenatural gas feed (“warm end temperature difference value”) and comparingsame with the actual warm end temperature difference;

means for setting a predetermined value of the temperature of a streamat a location between the liquefying and subcooling sections of the heatexchanger means (“mid-point temperature”) and comparing same with theactual mid-point temperature;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates and also, when thedifference between actual and predetermined warm end temperaturedifferences exceeds a threshold value, to said difference between warmend temperature differences, MRL flow rate;

means for varying the MRV flow rate to maintain an MRL/MRV ratio, whichratio is determined by the difference between the actual andpredetermined mid-point temperatures; and

means for varying, by an amount corresponding to the difference betweenthe difference between the actual and predetermined LNG temperatures,the actual LNG flow rate.

Also in accordance with an embodiment illustrated in FIG. 4, theinvention provides a method of maintaining at a predetermined flow ratevalue and at a predetermined temperature value the liquefied natural gas(“LNG”) outlet stream of a natural gas liquefaction using heat exchangemeans, having a warm end to which the natural gas is fed, a liquefyingsection in which the natural gas is liquefied, a subcooling section inwhich the liquefied natural gas is subcooled and a cold end from whichsaid LNG outlet stream is withdrawn, in which refrigeration duty isprovided in the liquefying section by a first refrigerant (“MRL”) cooledin said heat exchange means and supplied for refrigeration duty at anMRL flow rate and in the subcooling section by a second refrigerant(“MRV”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRV flow rate, comprising the steps of:

comparing said predetermined LNG flow rate value with the actual LNGflow rate;

comparing said predetermined LNG temperature value with the actual LNGtemperature;

comparing a predetermined value of the temperature difference betweenspent refrigerant leaving the warm end of the heat exchange means and astream entering said warm end selected from MRL, MRV and the natural gasfeed (“warm end temperature difference value”) with the actual warm endtemperature difference;

comparing a predetermined value of temperature of a stream at a locationbetween the liquefying and subcooling sections of the heat exchangermeans (“mid-point temperature”) with the actual mid-point temperature;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates and also, when the difference betweenactual and predetermined warm end temperature differences exceeds athreshold value, to said difference between warm end temperaturedifferences, the MRL flow rate;

varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio isdetermined by the difference between the actual and predeterminedmid-point temperatures; and

varying, by an amount corresponding to the difference between the actualand predetermined LNG temperatures, the actual LNG flow rate.

In accordance with a corresponding apparatus embodiment, the inventionprovides a control system for maintaining at a predetermined flow ratevalue and at a predetermined temperature value the liquefied natural gas(“LNG”) outlet stream of a natural gas liquefaction using heat exchangemeans, having a warm end to which the natural gas is fed, a liquefyingsection in which the natural gas is liquefied, a subcooling section inwhich the liquefied natural gas is subcooled and a cold end from whichsaid LNG outlet stream is withdrawn, in which refrigeration duty isprovided in the liquefying section by a first refrigerant (“MRL”) cooledin said heat exchange means and supplied for refrigeration duty at anMRL flow rate and in the subcooling section by a second refrigerant(“MRV”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRV flow rate, comprising:

means for comparing said predetermined LNG flow rate value with theactual LNG flow rate;

means for comparing said predetermined LNG temperature value with theactual LNG temperature;

means for comparing a predetermined value of the temperature differencebetween spent refrigerant leaving the warm end of the heat exchangemeans and a stream entering said warm end selected from MRL, MRV and thenatural gas feed (“warm end temperature difference value”) with theactual warm end temperature difference;

means for comparing a predetermined value of temperature of a stream ata location between the liquefying and subcooling sections of the heatexchanger means (“mid-point temperature”) with the actual mid-pointtemperature;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates and also, when thedifference between actual and predetermined warm end temperaturedifferences exceeds a threshold value, to said difference between warmend temperature differences, the MRL flow rate;

means for varying the MRV flow rate to maintain an MRL/MRV ratio, whichratio is determined by the difference between the actual andpredetermined mid-point temperatures; and

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG temperatures, the actual LNG flow rate.

In accordance with an embodiment illustrated in FIG. 5, the warm endtemperature difference is predetermined; the MRL flow rate is adjustedin response to the difference between actual and predetermined LNGproduct flow rates and hence the LNG/MRL ratio changed; the requiredMRL/MRV ratio is adjusted in response to the difference between theactual mid-point temperature and a calculated temperature determined bythe difference between the actual and predetermined warm end temperaturedifferences and the MRV flow rate adjusted to achieve that ratio; andthe actual flow rate is adjusted in response to the difference betweenactual and predetermined LNG product temperatures.

Thus, in accordance with an embodiment illustrated in FIG. 5, theinvention provides a method of maintaining at an adjustablepredetermined flow rate value and at an adjustable predeterminedtemperature value the liquefied natural gas (“LNG”) outlet stream of anatural gas liquefaction using heat exchange means, having a warm end towhich the natural gas is fed, a liquefying section in which the naturalgas is liquefied, a subcooling section in which the liquefied naturalgas is subcooled and a cold end from which said LNG outlet stream iswithdrawn, in which refrigeration duty is provided in the liquefyingsection by a first refrigerant (“MRL”) cooled in said heat exchangemeans and supplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising the steps of:

setting the predetermined flow rate value for the LNG outlet stream andcomparing said value with the actual LNG flow rate;

setting the predetermined temperature value for the LNG outlet streamand comparing said LNG temperature value with the actual LNGtemperature;

setting a predetermined value of the temperature difference betweenspent refrigerant leaving the warm end of the heat exchange means and astream entering said warm end selected from MRL, MRV and the natural gasfeed (“warm end temperature difference value”) and comparing said warmend temperature difference value with the actual warm end temperaturedifference;

comparing the temperature of a stream at a location between theliquefying and subcooling sections of the heat exchanger means(“mid-point temperature”) with a calculated temperature value, which isdetermined by the difference between the actual and predetermined actualwarm end temperature differences;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates, the MRL flow rate;

varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio isdetermined by the difference between the actual and calculated mid-pointtemperatures; and

varying, by an amount corresponding to the difference between the actualand predetermined LNG temperatures, the actual LNG flow rate.

In accordance with a corresponding apparatus embodiment, the inventionprovides a control system for maintaining at an adjustable predeterminedflow rate value and at an adjustable predetermined temperature value theliquefied natural gas (“LNG”) outlet stream of a natural gasliquefaction using heat exchange means, having a warm end to which thenatural gas is fed, a liquefying section in which the natural gas isliquefied, a subcooling section in which the liquefied natural gas issubcooled and a cold end from which said LNG outlet stream is withdrawn,in which refrigeration duty is provided in the liquefying section by afirst refrigerant (“MRL”) cooled in said heat exchange means andsupplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising:

means for setting the predetermined flow rate value for the LNG outletstream and comparing said value with the actual LNG flow rate;

means for setting the predetermined temperature value for the LNG outletstream and comparing said LNG temperature value with the actual LNGtemperature;

means for setting a predetermined value of the temperature differencebetween spent refrigerant leaving the warm end of the heat exchangemeans and a stream entering said warm end selected from MRL, MRV and thenatural gas feed (“warm end temperature difference value”) and comparingsaid warm end temperature difference value with the actual warm endtemperature difference;

means for comparing the temperature of a stream at a location betweenthe liquefying and subcooling sections of the heat exchanger means(“mid-point temperature”) with a calculated temperature value, which isdetermined by the difference between the actual and predetermined actualwarm end temperature differences;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates, the MRL flow rate;

means for varying the MRV flow rate to maintain an MRL/MRV ratio, whichratio is determined by the difference between the actual and calculatedmid-point temperatures; and

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG temperatures, the actual LNG flow rate.

Also in accordance with an embodiment illustrated in FIG. 5, theinvention provides a method of maintaining at a predetermined flow ratevalue and at a predetermined temperature value the liquefied natural gas(“LNG”) outlet stream of a natural gas liquefaction using heat exchangemeans, having a warm end to which the natural gas is fed, a liquefyingsection in which the natural gas is liquefied, a subcooling section inwhich the liquefied natural gas is subcooled and a cold end from whichsaid LNG outlet stream is withdrawn, in which refrigeration duty isprovided in the liquefying section by a first refrigerant (“MRL”) cooledin said heat exchange means and supplied for refrigeration duty at anMRL flow rate and in the subcooling section by a second refrigerant(“MRV”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRV flow rate, comprising the steps of:

comparing said predetermined LNG flow rate value with the actual LNGflow rate;

comparing said predetermined LNG temperature value with the actual LNGtemperature;

comparing a predetermined value of the temperature difference betweenspent refrigerant leaving the warm end of the heat exchange means and astream entering said warm end selected from MRL, MRV and the natural gasfeed (“warm end temperature difference value”) with the actual warm endtemperature difference;

comparing the temperature of a stream at a location between theliquefying and subcooling sections of the heat exchanger means(“mid-point temperature”) with a calculated temperature value, which isdetermined by the difference between the actual and predetermined actualwarm end temperature differences;

varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates, the MRL flow rate;

varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio isdetermined by the difference between the actual and calculated mid-pointtemperatures; and

varying, by an amount corresponding to the difference between the actualand predetermined LNG temperatures, the actual LNG flow rate.

In accordance with a corresponding apparatus embodiment, the inventionprovides a control system for maintaining at a predetermined flow ratevalue and at a predetermined temperature value the liquefied natural gas(“LNG”) outlet stream of a natural gas liquefaction using heat exchangemeans, having a warm end to which the natural gas is fed, a liquefyingsection in which the natural gas is liquefied, a subcooling section inwhich the liquefied natural gas is subcooled and a cold end from whichsaid LNG outlet stream is withdrawn, in which refrigeration duty isprovided in the liquefying section by a first refrigerant (“MRL”) cooledin said heat exchange means and supplied for refrigeration duty at anMRL flow rate and in the subcooling section by a second refrigerant(“MRV”) cooled in said heat exchange means and supplied forrefrigeration duty at an MRV flow rate, comprising:

means for comparing said predetermined LNG flow rate value with theactual LNG flow rate;

means for comparing said predetermined LNG temperature value with theactual LNG temperature;

means for comparing a predetermined value of the temperature differencebetween spent refrigerant leaving the warm end of the heat exchangemeans and a stream entering said warm end selected from MRL, MRV and thenatural gas feed (“warm end temperature difference value”) with theactual warm end temperature difference;

means for comparing the temperature of a stream at a location betweenthe liquefying and subcooling sections of the heat exchanger means(“mid-point temperature”) with a calculated temperature value, which isdetermined by the difference between the actual and predetermined actualwarm end temperature differences;

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates, the MRL flow rate;

means for varying the MRV flow rate to maintain an MRL/MRV ratio, whichratio is determined by the difference between the actual and calculatedmid-point temperatures; and

means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG temperatures, the actual LNG flow rate.

Referring to FIG. 1, natural gas is introduced via line 100 into thewarm end of a first tube side of a heat exchanger 112 in which is itliquefied and then subcooled before leaving the heat exchanger at thecold end. Refrigeration duty in the heat exchange is provided by a multicomponent refrigerant (“MR”) circulating in a closed loop. Spentrefrigerant from the heat exchanger is fed via line 144 to a compressor102 and the compressed refrigerant is partially condensed in a cooler104 before separation in a phase separator 106. The liquid phase (“MRL”)is fed via line 124 to a second tube side of the heat exchanger in whichit is cooled before being throttled in valve 132 and introduced into theshell side of the heat exchanger 112 below the cold bundle. The vaporphase (“MRV”) is fed via line 134 to a third tube side of the heatexchanger 112 in which it is cooled and then liquefied before beingthrottled in valve 138 and introduced into the shell side of the heatexchanger at the cold end. The liquid and condensed vapor portionsvaporize in the heat exchanger and combine to provide the refrigerantfeed to line 144.

The flow of LNG product is controlled by a valve 120 and the flows ofthe refrigerant portions to the heat exchanger are controlled by valves132 and 138 respectively.

The temperature of the LNG product is compared in temperature indicatorcontroller (“TIC”) 114 against the required product temperaturedetermined by an operator set point (SP). A signal proportionate to thedifference in actual and required temperature is sent from the TIC 114to a flow indicator controller (“FIC”) 116, which in turn adjusts theposition of the product valve 120 to maintain the required temperature.At constant refrigeration, an increase in product flow will reduce theactual product temperature and a decrease in product flow will reducethe actual product temperature. The product flow rate is monitored bythe FIC 116 and a signal proportionate to the actual value (“PV”) of theflow is sent from the FIC 116 to a FIC 122 for comparison with a setpoint value determined by the operator.

A signal proportionate to the difference between the actual and requiredproduct flow rates is sent to FIC 126, which compares the actual flowrate of the MRL with a required value set by that signal. The MRLcontrol valve 132 is adjusted in response to differences between theactual and required flow rates in order to adjust the refrigeration inheat exchanger 112.

A signal proportionate to the difference between actual and required MRLflow rates is sent to a flow ratio indicator controller (“FRIC”) 140where it is compared with a signal from flow indicator (“FI”) 136measuring actual MRV flow rate in order to determine the actual MRV/MRLflow ratio. The actual MRV/MRL flow rate is compared with a set pointvalue determined by a signal received from the temperature differentialindicator controller (“TDIC”) 142. A signal proportionate to thedifference between the actual and required MRV/MRL flow ratios adjustsflow valve 138 and the corresponding refrigeration provided to the heatexchanger 112.

The TDIC 142 compares the actual temperature difference between thespent refrigerant in line 144 and the MRL in line 124 with a set pointvalue determined by the operator. The set point signal provided by theTDIC 142 to the FRIC 140 is proportionate to that difference intemperature.

The TDIC 142 could measure temperature difference between the spentrefrigerant and either the MRV in line 134 or the natural gas feed inline 100 instead of the difference with the MRL as shown in FIG. 1.

FI 136 could be located upstream instead of downstream of the heatexchanger 112. Similarly, the FIC 126 also could be located upstreaminstead of downstream of the heat exchanger 112.

It will be apparent that following operator change to the required LNGproduct flow rate, required LNG product temperature and/or warm endtemperature difference (“WETD”), there will be resultant changes tovalves 132 and 138 determined by the extent to which the flow rate,temperature and/or WETD have been changed. This will change the amountof refrigeration provided to the heat exchanger 112 and thereby changethe difference between the actual and set LNG product temperaturevalues. That change will adjust the valve 120 and hence the actualproduct flow rate. The change of the actual product flow rate willresult in further adjustment valves 132 and 138 controlling therefrigeration supplied to the heat exchanger 112 and provide acorresponding change in the actual LNG product temperature.

Essentially simultaneously with actual change in product temperature,there will be a corresponding change in the WETD detected by TDIC 142that will result in a corresponding change in the required MRV/MRL flowratio of FRIC 140. Further, also essentially simultaneously with thechange in actual product flow rate, there will be a corresponding changein product temperature that will cause, via the change in differencebetween actual and required product flow rates, change in refrigeration.Thus, the differences between actual and required product temperatures,actual and required LNG product flow rates and actual and required WETDswill automatically incrementally change in order to achieve the requiredcombination of LNG product flow rate, LNG product temperature and WETD.Further, the control system will automatically change the refrigerationprovided to the heat exchanger to maintain the set values if there isany change in LNG product flow rate, LNG product temperature or WETDarising from changes to any of those parameters not occasioned bychanges to their required values, such as changes in NG composition, NGflow rate, partial condensation refrigeration duty for 104, ambient airtemperature, cooling water temperature, or atmospheric pressure.

The control system of FIG. 2 differs from that of FIG. 1 in that the LNGproduct valve 120 is adjusted in response to changes in the WETD and therequired MRV/MRL flow ratio is determined by the difference between theactual and required LNG product temperatures. In particular, the TDIC142 sends a signal to FIC 116 instead of to FRIC 140 and TIC 114 sends asignal to FRIC 140 instead of FIC 116.

The control system of FIG. 3 differs from that of FIG. 2 in that thesignal from TDIC 142 to FIC 116 is dependent upon the difference betweenthe actual and required MRL flow rates. In particular, a signalproportionate to that difference is sent to a multiplier 300 to modifythe signal from TDIC 142.

The control system of FIG. 4 differs from that of FIG. 1 in that therequired MRV/MRL flow ratio is determined by the difference betweenactual and required temperatures at a mid-point of the heat exchanger112 located between the liquefying and subcooling sections of the heatexchanger, typically between the cold and warm or middle bundles of theheat exchanger. A TIC 400 has a set point determined by the operator andcompares that set point with the actual mid-point temperature. Themid-point temperature can be that on the shell side of the heatexchanger 112 as shown in FIG. 4 or could be the temperature of the LNGor MRL or MRV at an appropriate location in the relevant tube section.In this embodiment adjustment of the MRL flow rate by valve 132 inresponse to the difference between the actual and required MRV flowrates is overridden by a FIC 326 responsive to the WETD if the actualdifference differs from the required difference by a predeterminedamount.

The control system of FIG. 5 differs from that of FIG. 4 in that therequired mid-point temperature is not operator set but is determined bythe difference between actual and required WETDs. In particular, asignal from TDIC 142 no longer provides an override to the FIC 126control of valve 132 but provides a set point for TIC 400.

The control system of FIG. 6 differs from that of FIG. 3 in that aconstraint controller 146 limits the opening of the MRV valve 138 to,for example 90%, by adjusting a “Production Factor” that is multipliedby multiplier 148 to produce an LNG Master Flow Controller set point.When the system is in control and the valve 138 is open less than theset 90% (or other predetermined maximum) amount the controller 146provides a Production Factor of 1 and does not limit production.However, if the valve position exceeds 90% (or other predeterminedmaximum) amount, then the controller would begin to reduce theProduction Factor until the system was in control with the requiredmaximum valve position.

It is a common feature of all of the exemplified embodiments that thereis no change in LNG product flow rate except in response to changes inrefrigeration duty for the heat exchanger 112.

EXAMPLES

Each of the embodiments of FIGS. 2 to 5 and the comparative process ofFIG. 7 was subject to the following disturbances:

-   -   Increase LNG rundown temperature from −247° F. (−155° C.) to        −245° F. (−153.9° C.) in 24 minutes;    -   Decrease production by 5% while simultaneously decreasing        turbine speed by 1% in 24 minutes; and    -   Increase production by 2.8% in 24 minutes.

The process of FIG. 7 differs from that of FIG. 1 in that the LNGproduct flow rate is directly adjusted to the required value and FIC 126is controlled by the difference between actual and desired LNG producttemperature difference

Response to Increase LNG Rundown Temperature

The process of FIG. 1 had some oscillations in its response to theincreased rundown temperature with both the LNG flow rate and the LNGtemperature oscillating somewhat as they reached the new steady state.It is believed that this was more a result of conflict between the twocontrollers rather than a tuning issue. The process of FIG. 7 did notoscillate but did take a long time to reach the new steady state, whichis believed to have been a function of tuning rather than controllerinteraction. The process of FIG. 2 exhibited much tighter control of thetemperature with little disturbance to the rest of the system. Theprocess of FIG. 3 showed similar ability to that of FIG. 2 in trackingthe LNG temperature set point and slightly less disturbances to the restof the system. Both of the processes of FIG. 2 and 3 showed the bestresponse to this disturbance.

Decrease LNG Production with Simultaneous Turbine Speed Reduction

The process of FIG. 1 had difficulty in following the LNG production setpoint taking 2 hours to return to steady state. During that time, theLNG temperature had deviations as large as 2° F. (1.1° C.) warmer beforefinally reaching steady state over 2 hours after the disturbance began.The process of FIG. 7, with its direct control over LNG flow rate hadexcellent tracking of the LNG production set point, but also saw largetemperature swings before finally reaching steady state almost 3 hoursafter the disturbance. The process of FIG. 2 lagged in the tracking ofthe LNG production set point taking two hours to reach steady state, butmaintained tight LNG temperature control throughout the disturbance. Theprocess of FIG. 3 tracked the set point of the LNG production very wellreaching steady state within an hour of the beginning of the disturbanceand maintaining tight temperature control throughout.

LNG Production Increase

The process of FIG. 1 took 2 hours to reach the desired set point aswell as allowing the LNG temperature to drift cold by 1° F. (0.55° C.)before returning to its set point. The process of FIG. 3 experienced amore than 1° F. (0.55° C.) warming of the LNG before returning to steadystate in close to 3 hours. The process of FIG. 2 took 3 hours to reachthe new production set point, but maintained excellent temperaturecontrol throughout. The process of FIG. 3 showed excellent response tothe production change and maintained tight temperature control.

Unattainable Production

Although the increased production disturbance was easily achieved by theexemplified embodiments of the invention, there still remained aquestion as to how the systems would respond to a truly unattainableproduction disturbance. Accordingly, the process of FIG. 3 was subjectedto a disturbance where the production set point was raised to 7% higherthan the current steady state. The simulation was also set up tosimulate bog down of the MR turbine 102 and additional parameters weremonitored to determine the systems responsiveness.

The system tracked the LNG production to the new set point, however, theLNG temperature continued to rise driving the MRV valve 138 fully open.The LNG temperature finally settled out approximately 4° F. (2.2° C.)warmer than the desired LNG temperature. The mid point temperaturebetween the middle and cold bundles of the heat exchanger 112 warmed upby close to 20° F. (11° C.). The increased MRV flow almost doubled thecold bundle pressure drop. The gas turbine reached full power and thenbogged down reducing its speed by approximately 1%. The results showedthat the control system had no checks to prevent it from reaching anunacceptable new operating point.

The process of FIG. 6 overcomes this problem by providing a check toprevent the control system from reaching an undesirable operating point.With the controller 146 set to limit the valve opening to 90%, the 7%production increase limited production to a 4.8% increase and maintainedcontrol of the LNG temperature throughout and did not bog down the MRturbine 102.

Other embodiments and benefits of the invention will be apparent tothose skilled in the art from a consideration of the specification andfrom practice of the invention disclosed herein. It is intended thatthis specification be considered as exemplary only with modificationsand variations being within the scope and spirit of the invention asdefined by the following claim. In particular, any of the exemplifiedembodiments could be used for liquefaction of gases other than naturalgas and the tube and shell heat exchanger 112 could be replaced by twoor more individual heat exchanges arranged in series and/or by any ofthe heat exchanger types known in the art.

1. A method of maintaining at a predetermined flow rate value and at apredetermined temperature value the liquefied gas (“IG”) outlet streamof a gas liquefaction in which a gas feed is liquefied by refrigerationin heat exchange means, comprising the steps of: comparing saidpredetermined LG flow rate value with the actual LG flow rate; comparingsaid predetermined LG temperature value with the actual LG temperature;and varying the refrigeration provided by said heat exchange means inresponse to said LG flow rate and LG temperature comparisons to reduceany differences, characterized in that variation of the refrigeration toreduce any LG temperature difference is initiated before variation ofthe LG flow rate to reduce any LG flow rate difference.
 2. The method ofclaim 1, wherein the gas feed is natural gas.
 3. The method of claim 1,wherein the actual LG flow rate is adjusted in response to changes inthe actual LG temperature.
 4. The method of claim 1, wherein the actualLG flow rate is adjusted in response to changes in the temperaturedifference between a stream entering the warm end of the heat exchangemeans and a stream leaving that end of the heat exchanger.
 5. The methodof claim 1, wherein the actual LG flow rate is adjusted in response tochanges in the temperature of a stream at a location between theliquefying and subcooling sections of the heat exchanger means(“mid-point temperature”).
 6. The method of claim 1 wherein saidpredetermined values are adjustable and the method further comprises thesteps of: setting the predetermined flow rate value for the LG outletstream and setting the predetermined temperature value for the LG outletstream.
 7. The method of claim 6, wherein the gas feed is natural gas.8. The method of claim 6, wherein the actual LG flow rate is adjusted inresponse to changes in the actual LG temperature.
 9. The method of claim6, wherein the actual LG flow rate is adjusted in response to changes inthe temperature difference between a stream entering the warm end of theheat exchange means and a stream leaving that end of the heat exchanger.10. The method of claim 6, wherein the actual LG flow rate is adjustedin response to changes in the temperature of a stream at a locationbetween the liquefying and subcooling sections of the heat exchangermeans (“mid-point temperature”).
 11. A method of maintaining at apredetermined flow rate value and at a predetermined temperature valuethe liquefied natural gas (“LNG”) outlet stream of a natural gasliquefaction using heat exchange means, having a warm end to which thenatural gas is fed, a liquefying section in which the natural gas isliquefied, a subcooling section in which the liquefied natural gas issubcooled and a cold end from which said LNG outlet stream is withdrawn,in which refrigeration duty is provided in the liquefying section by afirst refrigerant (“MRL”) cooled in said heat exchange means andsupplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising the steps of: comparing said predetermined LNG flow ratevalue with the actual LNG flow rate; comparing said predetermined LNGtemperature value with the actual LNG temperature; comparing apredetermined value of (i) the temperature difference between spentrefrigerant leaving the warm end of the heat exchange means and a streamentering said warm end selected from MRL, MRV and the natural gas feed(“warm end temperature difference”) or (ii) the temperature of a streamat a location between the liquefying and subcooling sections of the heatexchanger means (“mid-point temperature”) with the actual warm endtemperature difference or actual mid-point temperature respectively;varying, by an amount corresponding to the difference between the actualand predetermined LNG flow rates, one of the MRL and MRV flow rates;varying the other of the MRV and MRL flow rates to maintain an MRL/MRVratio, which ratio is determined by one of (a) the difference betweenthe actual and predetermined LNG temperatures and (b) the differencebetween the actual and predetermined warm end temperature differences ormid-point temperatures; and varying, by an amount corresponding to theother of (b) the difference between the actual and predetermined warmend temperature differences or mid-point temperatures and (a) thedifference between the actual and predetermined LNG temperatures, theactual LNG flow rate.
 12. The method of claim 11, wherein the MRL flowrate varies by an amount corresponding to the difference between theactual and predetermined LNG flow rates.
 13. The method of claim 11,wherein the MRL flow rate varies by an amount corresponding to thedifference between the actual and predetermined warm end temperaturedifferences.
 14. The method of claim 11, wherein the MRL flow ratevaries by an amount corresponding to the difference between the actualand predetermined mid-point temperatures.
 15. The method of claim 11,wherein the MRL/MRV ratio is determined by the difference between theactual and predetermined LNG temperatures.
 16. The method of claim 11,wherein the MRL/MRV ratio is determined by the difference between theactual and predetermined warm end temperature differences.
 17. Themethod of claim 11, wherein the MRL/MRV ratio is determined by thedifference between actual and predetermined mid-point temperatures. 18.A method of claim 11 wherein said predetermined values are adjustableand the method further comprises the steps of: setting the predeterminedflow rate value for the LNG outlet; setting the predeterminedtemperature value for the LNG outlet stream; and setting thepredetermined value of (i) the warm end temperature difference value or(ii) the mid-point temperature.
 19. The method of claim 18, wherein theMRL flow rate varies by an amount corresponding to the differencebetween the actual and predetermined LNG flow rates.
 20. The method ofclaim 18, wherein the MRL flow rate varies by an amount corresponding tothe difference between the actual and predetermined warm end temperaturedifferences.
 21. The method of claim 18, wherein the MRL flow ratevaries by an amount corresponding to the difference between the actualand predetermined mid-point temperatures.
 22. The method of claim 19,wherein the MRL/MRV ratio is determined by the difference between theactual and predetermined LNG temperatures.
 23. The method of claim 22,wherein the MRL/MRV ratio is determined by the difference between theactual and predetermined warm end temperature differences.
 24. Themethod of claim 22, wherein the MRL/MRV ratio is determined by thedifference between actual and predetermined mid-point temperatures. 25.A method of maintaining at a predetermined flow rate value and at apredetermined temperature value the liquefied natural gas (“LNG”) outletstream of a natural gas liquefaction using heat exchange means, having awarm end to which the natural gas is fed, a liquefying section in whichthe natural gas is liquefied, a subcooling section in which theliquefied natural gas is subcooled and a cold end from which said LNGoutlet stream is withdrawn, in which refrigeration duty is provided inthe liquefying section by a first refrigerant (“MRL”) cooled in saidheat exchange means and supplied for refrigeration duty at an MRL flowrate and in the subcooling section by a second refrigerant (“MRV”)cooled in said heat exchange means and supplied for refrigeration dutyat an MRV flow rate, comprising the steps of: comparing saidpredetermined LNG flow rate value with the actual LNG flow rate;comparing said predetermined LNG temperature value with the actual LNGtemperature; comparing a predetermined value of the temperaturedifference between spent refrigerant leaving the warm end of the heatexchange means and a stream entering said warm end selected from MRL,MRV and the natural gas feed (“warm end temperature difference”) withthe actual warm end temperature difference; varying, by an amountcorresponding to the difference between the actual and predetermined LNGflow rates, the MRL flow rate; varying the MRV flow rate to maintain anMRL/MRV ratio, which ratio is determined by difference between theactual and predetermined warm end temperature differences; and varying,by an amount corresponding to the difference between the actual andpredetermined LNG temperatures, the actual LNG flow rate.
 26. The methodof claim 25 wherein said predetermined values are adjustable and themethod further comprises the steps of: setting the predetermined flowrate value for the LNG outlet stream; setting the predeterminedtemperature value for the LNG outlet stream; and setting thepredetermined value of the warm end temperature difference value.
 27. Amethod of maintaining at a predetermined flow rate value and at apredetermined temperature value the liquefied natural gas (“LNG”) outletstream of a natural gas liquefaction using heat exchange means, having awarm end to which the natural gas is fed, a liquefying section in whichthe natural gas is liquefied, a subcooling section in which theliquefied natural gas is subcooled and a cold end from which said LNGoutlet stream is withdrawn, in which refrigeration duty is provided inthe liquefying section by a first refrigerant (“MRL”) cooled in saidheat exchange means and supplied for refrigeration duty at an MRL flowrate and in the subcooling section by a second refrigerant (“MRV”)cooled in said heat exchange means and supplied for refrigeration dutyat an MRV flow rate, comprising the steps of: comparing saidpredetermined LNG flow rate value with the actual LNG flow rate;comparing said predetermined LNG temperature value with the actual LNGtemperature; comparing a predetermined value of the temperaturedifference between spent refrigerant leaving the warm end of the heatexchange means and a stream entering said warm end selected from MRL,MRV and the natural gas feed (“warm end temperature difference”) withthe actual warm end temperature difference; varying, by an amountcorresponding to the difference between the actual and predetermined LNGflow rates, the MRL flow rate; varying the MRV flow rate to maintain anMRL/MRV ratio, which ratio is determined by the difference between theactual and predetermined LNG temperatures; and varying, by an amountcorresponding to the difference between the actual and predeterminedwarm end temperature differences, the actual LNG flow rate.
 28. Themethod of claim 27 wherein said predetermined values are adjustable andthe method further comprises the steps of: setting the predeterminedflow rate value for the LNG outlet stream; setting the predeterminedtemperature value for the LNG outlet stream; and setting thepredetermined value of the warm end temperature difference value.
 29. Amethod of maintaining at a predetermined flow rate value and at apredetermined temperature value the liquefied natural gas (“LNG”) outletstream of a natural gas liquefaction using heat exchange means, having awarm end to which the natural gas is fed, a liquefying section in whichthe natural gas is liquefied, a subcooling section in which theliquefied natural gas is subcooled and a cold end from which said LNGoutlet stream is withdrawn, in which refrigeration duty is provided inthe liquefying section by a first refrigerant (“MRL”) cooled in saidheat exchange means and supplied for refrigeration duty at an MRL flowrate and in the subcooling section by a second refrigerant (“MRV”)cooled in said heat exchange means and supplied for refrigeration dutyat an MRV flow rate, comprising the steps of: comparing saidpredetermined LNG flow rate value with the actual LNG flow rate;comparing said predetermined LNG temperature value with the actual LNGtemperature; comparing a predetermined value of the temperaturedifference between spent refrigerant leaving the warm end of the heatexchange means and a stream entering said warm end selected from MRL,MRV and the natural gas feed (“warm end temperature difference”) withthe actual warm end temperature difference; varying, by an amountcorresponding to the difference between the actual and predetermined LNGflow rates, the MRL flow rate; varying the MRV flow rate to maintain anMRL/MRV ratio, which ratio is determined by the difference between theactual and predetermined LNG temperatures; and varying, by an amountcorresponding to the difference between the actual and predeterminedwarm end temperature differences multiplied by a value dependent on theactual MRL flow rate, the actual LNG flow rate.
 30. The method of claim29 wherein said predetermined values are adjustable and the methodfurther comprises the steps of: setting the predetermined flow ratevalue for the LNG outlet stream; setting the predetermined temperaturevalue for the LNG outlet stream; and setting the predetermined value ofthe warm end temperature difference value.
 31. A method of maintainingat a predetermined flow rate value and at a predetermined temperaturevalue the liquefied natural gas (“LNG”) outlet stream of a natural gasliquefaction using heat exchange means, having a warm end to which thenatural gas is fed, a liquefying section in which the natural gas isliquefied, a subcooling section in which the liquefied natural gas issubcooled and a cold end from which said LNG outlet stream is withdrawn,in which refrigeration duty is provided in the liquefying section by afirst refrigerant (“MRL”) cooled in said heat exchange means andsupplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising the steps of: comparing said predetermined LNG flow ratevalue with the actual LNG flow rate; comparing said predetermined LNGtemperature value with the actual LNG temperature; comparing apredetermined value of the temperature difference between spentrefrigerant leaving the warm end of the heat exchange means and a streamentering said warm end selected from MRL, MRV and the natural gas feed(“warm end temperature difference”) with the actual warm end temperaturedifference; comparing a predetermined value of temperature of a streamat a location between the liquefying and subcooling sections of the heatexchanger means (“mid-point temperature”) with the actual mid-pointtemperature; varying, by an amount corresponding to the differencebetween the actual and predetermined LNG flow rates and also, when thedifference between actual and predetermined warm end temperaturedifferences exceeds a threshold value, to said difference between warmend temperature differences, the MRL flow rate; varying the MRV flowrate to maintain an MRL/MRV ratio, which ratio is determined by thedifference between the actual and predetermined mid-point temperatures;and varying, by an amount corresponding to the difference between theactual and predetermined LNG temperatures, the actual LNG flow rate. 32.The method of claim 31 wherein said predetermined values are adjustableand the method further comprises the steps of: setting the predeterminedflow rate value for the LNG outlet stream; setting the predeterminedtemperature value for the LNG outlet stream; and setting thepredetermined warm end temperature difference value.
 33. A method ofmaintaining at a predetermined flow rate value and at a predeterminedtemperature value the liquefied natural gas (“LNG”) outlet stream of anatural gas liquefaction using heat exchange means, having a warm end towhich the natural gas is fed, a liquefying section in which the naturalgas is liquefied, a subcooling section in which the liquefied naturalgas is subcooled and a cold end from which said LNG outlet stream iswithdrawn, in which refrigeration duty is provided in the liquefyingsection by a first refrigerant (“MRL”) cooled in said heat exchangemeans and supplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising the steps of: comparing said predetermined LNG flow ratevalue with the actual LNG flow rate; comparing said predetermined LNGtemperature value with the actual LNG temperature; comparing apredetermined value of the temperature difference between spentrefrigerant leaving the warm end of the heat exchange means and a streamentering said warm end selected from MRL, MRV and the natural gas feed(“warm end temperature difference”) with the actual warm end temperaturedifference; comparing the temperature of a stream at a location betweenthe liquefying and subcooling sections of the heat exchanger means(“mid-point temperature”) with a calculated temperature value, which isdetermined by the difference between the actual and predetermined warmend temperature differences; varying, by an amount corresponding to thedifference between the actual and predetermined LNG flow rates, the MRLflow rate; varying the MRV flow rate to maintain an MRL/MRV ratio, whichratio is determined by the difference between the actual and calculatedmid-point temperatures; and varying, by an amount corresponding to thedifference between the actual and predetermined LNG temperatures, theactual LNG flow rate.
 34. The method of claim 33 wherein saidpredetermined values are adjustable and the method further comprises thesteps of: setting the predetermined flow rate value for the LNG outletstream; setting the predetermined temperature value for the LNG outletstream; and setting the predetermined value of the warm end temperaturedifference value.
 35. A control system for maintaining at an adjustablepredetermined flow rate value and at an adjustable predeterminedtemperature value the liquefied gas (“LG”) outlet stream of a gasliquefaction in which a gas feed is liquefied by refrigeration in heatexchange means, comprising: means for comparing the predetermined flowrate value for the LG outlet stream and comparing said value with theactual LG flow rate; means for varying the actual LG product flow rate;means for comparing the predetermined temperature value for the LGoutlet stream and comparing said LNG temperature value with the actualLG temperature; and means for varying the refrigeration provided by saidheat exchange means in response to said LG flow rate and LG temperaturecomparisons to reduce any differences, characterized in that adjustmentof the means for varying the actual LG product flow rate is notinitiated until the refrigeration has been adjusted to reduce any LGtemperature difference.
 36. A control system for maintaining at apredetermined flow rate value and at a predetermined temperature valuethe liquefied natural gas (“LNG”) outlet stream of a natural gasliquefaction using heat exchange means, having a warm end to which thenatural gas is fed, a liquefying section in which the natural gas isliquefied, a subcooling section in which the liquefied natural gas issubcooled and a cold end from which said LNG outlet stream is withdrawn,in which refrigeration duty is provided in the liquefying section by afirst refrigerant (“MRL”) cooled in said heat exchange means andsupplied for refrigeration duty at an MRL flow rate and in thesubcooling section by a second refrigerant (“MRV”) cooled in said heatexchange means and supplied for refrigeration duty at an MRV flow rate,comprising the steps of: means for comparing the predetermined flow ratevalue for the LNG outlet stream and comparing said value with the actualLNG flow rate; means for varying the actual LG product flow rate; meansfor comparing the predetermined temperature value for the LNG outletstream and comparing said LNG temperature value with the actual LNGtemperature; means for comparing a predetermined value of (i) thetemperature difference between spent refrigerant leaving the warm end ofthe heat exchange means and a stream entering said warm end selectedfrom MRL, MRV and the natural gas feed (“warm end temperature differencevalue”) or (ii) the temperature of a stream at a location between theliquefying and subcooling sections of the heat exchanger means(“mid-point temperature”) and comparing same with the actual warm endtemperature difference or actual mid-point temperature respectively;means for varying, by an amount corresponding to the difference betweenthe actual and predetermined LNG flow rates, one of the MRL and MRV flowrates; means for varying the other of the MRV and MRL flow rates tomaintain an MRL/MRV ratio, which ratio is determined by one of (a) thedifference between the actual and predetermined LNG temperatures and (b)the difference between the actual and predetermined warm end temperaturedifferences or mid-point temperatures; and means for varying, by anamount corresponding to the other of (b) the difference between theactual and predetermined warm end temperature differences or mid-pointtemperatures and (a) the difference between the actual and predeterminedLNG temperatures, the actual LNG flow rate.